Apart from that, in many cases the Calc function is not the optimal way
to simplify an expression, since it tries to do a lot of things that
might not be needed at all. For example, for your expression, Contract
alone is sufficient:

returns I Sqrt[A], i.e. Mathematica assumes that Sqrt[-a] is
Sqrt[-a+ i eps]. If however, you actually meant it to be Sqrt[-a- i
eps], the result will be wrong. For this reason it is useful to add
a small imaginary part, to prevent Mathematica from making too many
assumptions. With